1. Field of the Invention
The present invention relates to a pyrometer, more particularly, it relates to a pyrometer for measuring the temperature of a target through light as a medium.
2. Description of the Prior Art
In such a pyrometer, it is necessary to know the emissivity of a target to be measured, but in general, such emissivity can not be known. Therefore, in a number of previously proposed pyrometers, the emissivities of a target to be measured are assumed as follows: In Japanese Laid-Open Patent Application No. 130622/1981, light having two different colors (two different wavelengths .lambda..sub.1, .lambda..sub.2) is measured, and the emissivity .epsilon. in the wavelength .lambda..sub.1 is assumed to be equal to the emissivity .epsilon. in the wavelength .lambda..sub.2. In the other Japanese Laid-Open Patent Application No. 130623/1981, light having three different colors (three different wavelengths) is measured, and the emissivities .epsilon.(.lambda.) in respective wavelengths are assumed that .epsilon.(.lambda.)=exp. (a.sub.0 +a.sub.1 .lambda.), wherein a.sub.0 and a.sub.1 represent constants respectively. Furthermore, in the U.S. Pat. No. 4,411,519, three temperatures T.sub.12, T.sub.23, and T.sub.31 are calculated with assuming .epsilon.(.lambda..sub.1)=.epsilon.(.lambda..sub.2), .epsilon.(.lambda..sub.2)=.epsilon.(.lambda..sub.3), .epsilon.(.lambda..sub.3)=.epsilon.(.lambda..sub.1) respectively, and a true temperature T is obtained by T=(T.sub.12 +T.sub.23 +T.sub.31)/3.
Meanwhile, in the other Japanese Laid-Open Patent Application No. 30727/1986, the intensities L(.lambda..sub.1) and L(.lambda..sub.2) of light reflected by the target are measured in two different wavelengths .lambda..sub.1, .lambda..sub.2 respectively, and the temperature is calculated on the basis of the measured light intensities L(.lambda..sub.1), L(.lambda..sub.2), and a reflected light intensity ratio L(.lambda..sub.1)/L(.lambda..sub.2) represented by the emissivities .epsilon.(.lambda..sub.1), .epsilon.(.lambda..sub.2) as follows: EQU L(.lambda..sub.1)/L(.lambda..sub.2)={1-.epsilon.(.lambda..sub.1)}/{1-.epsil on.(.lambda..sub.2)} (A).
Namely, assuming that the measured radiation intensity in wavelength .lambda. is D(.lambda.) and the radiation intensity of blackbody in the wavelength .lambda. at temperature T is D.sub.0 (.lambda., T), the following equation is established: EQU .epsilon.(.lambda.)=D(.lambda.)/D.sub.0 (.lambda., T) (B).
Here, the temperature T can be calculated in accordance with the equation (A), since the unknown factor is only T in the equation (A) if .epsilon.(.lambda.) shown in the equation (B) is substituted to the equation (A). D.sub.0 (.lambda., T) can be calculated in accordance with the well-known Planck formula and constants peculiar to the device.
However, in Japanese Patent Laid-Open Nos. 130622/1981 and 7529/1982, since it is assumed that .epsilon.(.lambda..sub.1)=.epsilon.(.lambda..sub.2), the true temperature can be calculated only when a spectral characteristic of emissivity is constant. In other words, when the spectral characteristic of emissivity is not constant, the assumption .epsilon.(.lambda..sub.1)=.epsilon.(.lambda..sub.2) itself is erroneous, so that the true temperature can not be calculated. In either case, in Japanese Patent Laid-Open No. 130622/1981, U.S. Pat. No. 4,411,519 and including No. 130623/1981, since information on the reflected light is not included in the assumption of an emissivity, the true temperature can not be obtained except in the case of constant spectral characteristic of emissivity.
On the other hand, in the calculation in Japanese Patent Laid-Open No. 30727/1986, there may be the case that two different temperatures are obtained. That is, in accordance with this prior art reference, the temperature calculation is performed by utilizing an inverse of the Planck formula. In general, utilization of the inverse of the Planck formula, however, results in undesirably deriving two solutions or no solution.